Phát triển xúc tác sử dụng vật liệu có cấu trúc nano. Tổng quan về vật liệu NANO I. Vật liệu nano là gì? Vật liệu nano (nano materials) là một trong những lĩnh vực nghiên cứu đỉnh cao sôi động nhất trong thời gian gần đây. Điều đó được thể hiện bằng số các công trình khoa học, số các bằng phát minh sáng chế , số các công ty có liên quan đến khoa học, công nghệ nano gia tăng theo cấp số mũ. Con số ước tính về số tiền đầu tư vào lĩnh vực này lên đến 8,6 tỷ đô la vào năm 2004 . Vậy thì tại sao vật liệu nano lại thu hút được nhiều đầu tư về tài chính và nhân lực đến vậy? Bài này sẽ điểm sơ qua về vật liệu nano, các phương pháp chế tạo, tính chất lí hóa, và các ứng dụng của chúng. Khi ta nói đến nano là nói đến một phần tỷ của cái gì đó, ví dụ, một nano giây là một khoảng thời gian bằng một phần tỷ của một giây. Còn nano mà chúng ta dùng ở đây có nghĩa là nano mét, một phần tỷ của một mét. Nói một cách rõ hơn là vật liệu chất rắn có kích thước nm vì yếu tố quan trọng nhất mà chúng ta sẽ làm việc là vật liệu ở trạng thái rắn. Vật liệu nano là một thuật ngữ rất phổ biến, tuy vậy không phải ai cũng có một khái niệm rõ ràng về thuật ngữ đó. Để hiểu rõ khái niệm vật liệu nano, chúng ta cần biết hai khái niệm có liên quan là khoa học nano (nanoscience) và công nghệ nano (nanotechnology). Theo Viện hàn lâm hoàng gia Anh quốc thì : Khoa học nano là ngành khoa học nghiên cứu về các hiện tượng và sự can thiệp (manipulation) vào vật liệu tại các quy mô nguyên tử, phân tử và đại phân tử. Tại các quy mô đó, tính chất của vật liệu khác hẳn với tính chất của chúng tại các quy mô lớn hơn. Công nghệ nano là việc thiết kế, phân tích đặc trưng, chế tạo và ứng dụng các cấu trúc, thiết bị, và hệ thống bằng việc điều khiển hình dáng và kích thước trên quy mô nano mét. Vật liệu nano là đối tượng của hai lĩnh vực là khoa học nano và công nghệ nano, nó liên kết hai lĩnh vực trên với nhau. Kích thước của vật liệu nano trải một khoảng khá rộng, từ vài nm đến vài trăm nm. Để có một con số dễ hình dung, nếu ta có một quả cầu có bán kính bằng quả bóng bàn thì thể tích đó đủ để làm ra rất nhiều hạt nano có kích thước 10 nm, nếu ta xếp các hạt đó thành một hàng dài kế tiếp nhau thì độ dài của chúng bằng một ngàn lần chu vi của trái đất. II. Tại sao vật liệu nano lại có các tính chất thú vị? Tính chất thú vị của vật liệu nano bắt nguồn từ kích thước của chúng rất nhỏ bé có thể so sánh với các kích thước tới hạn của nhiều tính chất hóa lí của vật liệu. Chỉ là vấn đề kích thước thôi thì không có gì đáng nói, điều đáng nói là kích thước của vật liệu nano đủ nhỏ để có thể so sánh với các kích thước tới hạn của một số tính chất (bảng 1). Vật liệu nano nằm giữa tính chất lượng tử của nguyên tử và tính chất khối của vật liệu. Đối với vật liệu khối, độ dài tới hạn của các tính chất rất nhỏ so với độ lớn của vật liệu, nhưng đối với vật liệu nano thì điều đó không đúng nên các tính chất khác lạ bắt đầu từ nguyên nhân này. Chúng ta hãy lấy một ví dụ trong bảng 1. Vật liệu sắt từ được hình thành từ những đô men, trong lòng một đô men, các nguyên tử có từ tính sắp xếp song song với nhau nhưng lại không nhất thiết phải song song với mô men từ của nguyên tử ở một đô men khác. Giữa hai đô men có một vùng chuyển tiếp được gọi là vách đô men. Độ dày của vách đô men phụ thuộc vào bản chất của vật liệu mà có thể dày từ 10100 nm. Nếu vật liệu tạo thành từ các hạt chỉ có kích thước bằng độ dày vách đô men thì sẽ có các tính chất khác hẳn với tính chất của vật liệu khối vì ảnh hưởng của các nguyên tử ở đô men này tác động lên nguyên tử ở đô men khác. III. Chế tạo vật liệu nano như thế nào? Các vật liệu nano có thể thu được bằng bốn phương pháp phổ biến, mỗi phương pháp đều có những điểm mạnh và điểm yếu, một số phương pháp chỉ có thể được áp dụng với một số vật liệu nhất định mà thôi. 1) Phương pháp hóa ướt (wet chemical) Bao gồm các phương pháp chế tạo vật liệu dùng trong hóa keo (colloidal chemistry), phương pháp thủy nhiệt, solgel, và kết tủa. Theo phương pháp này, các dung dịch chứa ion khác nhau được trộn với nhau theo một tỷ phần thích hợp, dưới tác động của nhiệt độ, áp suất mà các vật liệu nano được kết tủa từ dung dịch. Sau các quá trình lọc, sấy khô, ta thu được các vật liệu nano. Ưu điểm của phương pháp hóa ướt là các vật liệu có thể chế tạo được rất đa dạng, chúng có thể là vật liệu vô cơ, hữu cơ, kim loại. Đặc điểm của phương pháp này là rẻ tiền và có thể chế tạo được một khối lượng lớn vật liệu. Nhưng nó cũng có nhược điểm là các hợp chất có liên kết với phân tử nước có thể là một khó khăn, phương pháp solgel thì không có hiệu suất cao. 2) Phương pháp cơ học (mechanical) Bao gồm các phương pháp tán, nghiền, hợp kim cơ học. Theo phương pháp này, vật liệu ở dạng bột được nghiền đến kích thước nhỏ hơn. Ngày nay, các máy nghiền thường dùng là máy nghiền kiểu hành tinh hay máy nghiền quay. Phương pháp cơ học có ưu điểm là đơn giản, dụng cụ chế tạo không đắt tiền và có thể chế tạo với một lượng lớn vật liệu. Tuy nhiên nó lại có nhược điểm là các hạt bị kết tụ với nhau, phân bố kích thước hạt không đồng nhất, dễ bị nhiễm bẩn từ các dụng cụ chế tạo và thường khó có thể đạt được hạt có kích thước nhỏ. Phương pháp này thường được dùng để tạo vật liệu không phải là hữu cơ như là kim loại. 3) Phương pháp bốc bay Gồm các phương pháp quang khắc (lithography), bốc bay trong chân không (vacuum deposition) vật lí, hóa học. Các phương pháp này áp dụng hiệu quả để chế tạo màng mỏng hoặc lớp bao phủ bề mặt tuy vậy người ta cũng có thể dùng nó để chế tạo hạt nano bằng cách cạo vật liệu từ đế. Tuy nhiên phương pháp này không hiệu quả lắm để có thể chế tạo ở quy mô thương mại. 4) Phương pháp hình thành từ pha khí (gasphase) Gồm các phương pháp nhiệt phân (flame pyrolysis), nổ điện (electroexplosion), đốt laser (laser ablation), bốc bay nhiệt độ cao, plasma. Nguyên tắc của các phương pháp này là hình thành vật liệu nano từ pha khí. Nhiệt phân là phương pháp có từ rất lâu, được dùng để tạo các vật liệu đơn giản như carbon, silicon. Phương pháp đốt laser thì có thể tạo được nhiều loại vật liệu nhưng lại chỉ giới hạn trong phòng thí nghiệm vì hiệu suất của chúng thấp. Phương pháp plasma một chiều và xoay chiều có thể dùng để tạo rất nhiều vật liệu khác nhau nhưng lại không thích hợp để tạo vật liệu hữu cơ vì nhiệt độ của nó có thể đến 9000 C. Phương pháp hình thành từ pha khí dùng chủ yếu để tạo lồng carbon (fullerene) hoặc ống carbon, rất nhiều các công ty dùng phương pháp này để chế tạo mang tính thương mại.
Trang 1Recent developments in catalysis using nanostructured materials
Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA
Contents
1 Introduction 1
2 Alkylation 2
3 Dehydrogenation and hydrogenation 4
3.1 Dehydrogenation 5
3.2 Hydrogenation 7
4 Selective oxidation 10
4.1 Selective oxidation catalysis by nanosized gold and other noble metals 10
4.2 Selective oxidation of lower alkanes by bulk mixed metal oxides 12
4.3 Catalytic behavior of Mo-V-(Te-Nb)-O M1 phase catalysts 13
4.4 On cooperation of M1 and M2 phases in propane ammoxidation 13
4.5 Surface termination of M1 phase 13
5 Conclusions and future trends 14
Acknowledgements 16
References 16
1 Introduction
Heterogeneous catalysts enable many chemical transforma-tions of fossil resources (natural gas, methane, liquid petroleum, coal, etc.) into useful products[1,2] Catalysts are responsible for the production of over 60% of all chemicals and are used in some 90% of all chemical processes worldwide[3,4] According to a 2002
A R T I C L E I N F O
Article history:
Received 31 August 2008
Received in revised form 20 November 2008
Accepted 24 November 2008
Available online 14 December 2008
Keywords:
Heterogeneous catalysis
Nanostructured materials
Alkylation
Dehydrogenation
Hydrogenation
Selective oxidation
A B S T R A C T This review describes recent developments of size-, shape-, structure- and composition-dependent behavior of catalyst nanoparticles employed in alkylation, dehydrogenation, hydrogenation, and selective oxidation reactions for the conversion of hydrocarbons (with main emphasis on fossil resources) to chemicals Innovation in these areas is largely driven by novel synthesis of (nano)porous and nanostructured catalytic materials In case of alkylation, several new classes of porous materials have recently emerged as catalysts while the discovery of novel ultralarge-pore frameworks with desirable acidity remains largely a serendipitous process Noble metal nanoparticles such as Pt, Pd, Rh,
Au and their alloys with other metals have been extensively employed to catalyze a wide range of dehydrogenation, hydrogenation, and selective oxidation reactions of organic molecules Novel approaches are still required to synthesize and characterize stable gold and other metal nanoparticles with tightly controlled sizes to further advance the knowledge of their unique size-dependent catalytic behavior The bulk mixed metal oxides of vanadium, molybdenum, and other transition metals, such as the M1 phase for propane ammoxidation to acrylonitrile, have shown great promise as highly active and selective oxidation catalysts However, fundamental understanding of surface molecular structure– reactivity relationships of these systems remains highly limited Future advances in all these areas may
be possible through combined experimental and theoretical approaches
ß2008 Elsevier B.V All rights reserved
* Corresponding author Fax: +1 513 556 3473.
E-mail address: vguliant@alpha.che.uc.edu (V.V Guliants).
Contents lists available atScienceDirect
Applied Catalysis A: General
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / a p c a t a
0926-860X/$ – see front matter ß 2008 Elsevier B.V All rights reserved.
Trang 2article discussing the impact of catalysis on the U.S economy,
‘‘one-third of material gross national product in the U.S involves a
catalytic process somewhere in the production chain’’[5] Catalyst
manufacturing alone accounts for over $10 billion in sales
worldwide in four major sectors: refining, chemicals,
polymeriza-tion, and exhaust emission catalysts However, the value derived
from catalyst sales is greatly eclipsed by the total value of the
products that are produced Products produced from fossil
resources include chemical intermediates, polymers, plastic
packaging, paints, cleaning products, pesticides, sweeteners,
cosmetics, antibiotics, pharmaceuticals, and, of course, fuels The
global annual impact of catalysis is estimated to be $10 trillion[3]
As we look to the future, heterogeneous catalysis increasingly
holds the key to ‘‘green chemistry’’ and the promise of eliminating
or at least dramatically curbing pollution from chemical and
refining processes[6], through atomically tailoring the structure of
active and selective reaction sites in order to convert reactants
directly to products without generating by-products that typically
end up as harmful emissions[7]or as wastes[8]
The majority of industrial catalysts contain an active
compo-nent in the form of nanoparticles <20 nm in size that are dispersed
onto high-surface-area supports The importance of nanoparticles
and nanostructure to the performance of catalysts has stimulated
wide efforts to develop methods for their synthesis and
characterization, making this area of study an integral part of
nanoscience[9] This review provides some recent examples from
the literature of how catalysis performance (i.e., activity and
selectivity) is affected by the local size, shape, structure, and
composition of catalyst particles employed in four major classes of
organic reactions that underlie conversion of mainly fossil
resources to chemicals These four classes are alkylation,
dehy-drogenation, hydehy-drogenation, and selective oxidation The review
also highlights future trends in catalysis by nanostructured
materials that are expected to result in detailed understanding
of the effects of particle composition, size, and structure on catalyst
performance, which will support better green chemistry for the
future Ultimately, the goal of studies in catalysis by
nanostruc-tured materials is to develop an understanding of these complex
systems to a point where it will be possible to successfully exploit
nanoscale phenomena by design to create new heterogeneous
catalysts and green catalytic processes
2 Alkylation
The alkylation of aromatic compounds is widely used in the
large-scale synthesis of petrochemicals, fine chemicals, and
intermediates[10] This reaction consists of the replacement of
a hydrogen atom of an aromatic compound by an alkyl group
derived from an alkylating agent If the hydrogen being replaced is
on the aromatic ring, the reaction is electrophilic substitution,
which requires an acid catalyst If the hydrogen being replaced is
on the side chain of an aromatic molecule, then base catalysts or
radical conditions are needed
Acid catalysts used for alkylation of aromatic hydrocarbons are
Brønsted acids containing acidic protons These typically have
included acidic halides such as AlCl3 and BF3; protonic acids,
especially sulfuric acid, hydrofluoric acid, and phosphoric acid;
acidic oxides and zeolites; and organic cation exchange resins The
acidic halides and protonic acids are being rapidly replaced for
large-scale applications by solid alkylation catalysts, especially
zeolites, because these are much more desirable for environmental
reasons They are noncorrosive and offer additional advantages for
controlling the selectivity via their shape-selective properties This
brief overview focuses on recent developments related to the
synthesis and applications of novel solid catalysts in the alkylation
of aromatic compounds in which the nanoscale pore environment
of a solid catalyst is exploited to achieve superior activity and shape selectivity
The periodic mesoporous organosilicas (PMOs) represent one of the recent breakthroughs in the field of materials chemistry of mesoporous nanostructured materials[11] PMOs are synthesized from the bridged organosilane precursors, (R0O)3SiR(OR0)3 and possess some unique properties that cannot be obtained by other approaches, such as uniformly distributed organic groups in the mesoporous framework with maximum loading of 100% Unlike fully inorganic variants of ordered mesoporous frameworks, PMOs exhibit a periodic arrangement of the hydrophobic organic layers (e.g., ethylene and benzene) and hydrophilic silica layers within the pore walls (Fig 1)[12–14] Incorporation of heteroatoms in these PMOs provides a new approach for the synthesis of bifunctional mesoporous materials with crystal-like pore wall structure and novel catalytic properties in epoxidation (Ti), ammoxidation (Ru), and alkylation (Al) reactions [15 and references therein]
In studies where tetrahedral Al species were incorporated into the PMO frameworks during synthesis of ethylene-[12,13]and benzene-bridged PMOs[15], the resultant PMO phases with both mesoscale (2.3–3.0 nm pore diameters, 4.6–5.1 nm d1 0 0-spacings, and 2.1–2.6 nm thick walls) and molecular-scale periodicity possessed significantly improved hydrothermal stability, due to increased hydrophobic character and the incorporation of tetra-hedral aluminum in the mesoporous framework It was found that the ethylene-bridged mesoporous framework generated a greater amount of acid sites than the phenylene-bridged network with a similar Si/Al ratio and aluminum coordination environment Consequently, the Al-containing ethylene-bridged PMO exhibited higher catalytic activity in alkylation of 2,4-di-tert-butylphenol with cinnamyl alcohol under identical reaction conditions[14] Moreover, periodic arrangement of organic layers in these materials offers a possibility to introduce several types of functional groups and even control their relative spacing on the mesopore surface, which together with metal substitution into their silica framework component may offer a new strategy for controlling multistep catalytic reactions in a single vessel Microporous zeolites have been widely used in industry as solid-acid catalysts for a variety of alkylation reactions Significant mass transport limitations to and from the active sites located in micropores severely limit their performance[16] To overcome these limitations, various strategies have been successfully pursued, such as the synthesis of nanosized zeolites (early work
on colloidal zeolites Y and A by Schoeman et al.[17]), ultralarge-pore zeolites and zeolite analogs VPI-5, JDF-20, UTD-1, CIT-5,
SSZ-53, ECR-34, ITQ-21, etc.[18]and ordered mesoporous materials MCM-41, SBA-15, FSM-16, etc.[19] However, the use of these materials is rather limited, due to the difficulty of separating nanosized zeolite crystals from the reaction mixture, the complex-ity of the templates used for the synthesis of ultralarge-pore zeolites, and the relatively low thermal and hydrothermal stability
of ordered mesoporous materials More recently, mesoporous zeolites from nanosized carbon templates have also been reported
[20], but their industrial applications are still limited by the complexity of the synthetic procedure involved and the hydro-phobicity of the carbon templates
Recently, hierarchical mesoporous zeolite beta (Beta-H) tem-plated from a mixture of small organic ammonium salts and mesoscale cationic polymers has been reported that possesses dual porosity due to the presence of zeolite nanocrystals forming a mesoporous structure, which provides significantly improved mass transport behavior in alkylation catalysis [21] This route involves a one-step hydrothermal synthesis, and the templated mixture is homogeneously dispersed in the synthetic gel Importantly, these novel hierarchical zeolites exhibit excellent
Trang 3catalytic properties as compared to conventional zeolite beta Beta
zeolite is generally synthesized from a small organic template of
tetraethylammonium hydroxide (TEAOH) Hierarchical
mesopor-ous Beta zeolite (Beta-H) is crystallized in the presence of TEAOH
and a mesoscale cationic polymer, polydiallyldimethylammonium
chloride (PDADMAC)
Scanning electron microscopy and transmission electron
micro-scopy (SEM and TEM) images of Beta-H reveal the presence of zeolite
particles about 600 nm in size and hierarchical mesoporosity in the
5–40 nm range Partial connections may be observed between these
hierarchical pores that are beneficial for the mass transfer of
reactants and products in catalysis Beta-H showed a high activity
and selectivity as an alkylation catalyst, as well as a long catalyst life
relative to the sample of conventional Beta zeolite The similarities of
Beta-H to conventional Beta zeolite in terms of Si/Al ratios,
aluminum distribution, and acidic strength, as well as the larger
particle size of Beta-H than that of Beta zeolite indicate that the
higher catalytic activity of Beta-H in the model alkylation reaction is
related to the hierarchical mesoporosity in Beta-H, which is
important for improving the mass transport of the reactants and
products in the alkylation of benzene with propan-2-ol
The presence of hierarchical mesoporosity in the Beta-H sample
is attributed to the use of the molecular and aggregated cationic
polymer PDADMAC The molecular weight of the cationic polymer
lies in the range 105–106, and its size is estimated at 5–40 nm,
which is in good agreement with the dimensions of the mesopores
obtained from high-resolution TEM studies The cationic polymers
could effectively interact with negatively charged inorganic silica
species in alkaline media, resulting in the hierarchical mesopor-osity The addition of a greater amount of cationic polymer in the synthetic gel yields Beta zeolite with larger mesoporosity, indicating the controllable mesoporosity of the zeolite sample The synthesis of hierarchical mesoporous zeolites is not limited to the Beta variety, which was obtained through use of TEAOH with PDADMAC, but other mixtures of organic amine salts and cationic polymer templates may be used if they effectively interact with inorganic species in alkaline media under conditions to crystallize the zeolites For example, hierarchical mesoporous ZSM-5 zeolite (ZSM-5-H) was obtained using a mixture of tetrapropylamine hydroxide and dimethyldiallyl ammonium chloride acrylamide copolymer (10 wt.%) Particularly, the use of the hierarchical mesoporous ZSM-5-H in the catalytic cracking of 1,3,5-triisopro-pylbenzene showed that it is much more active catalyst than conventional ZSM-5 under the same reaction conditions[21]
A novel ultralarge-pore silicogermanate zeolite ITQ-33 was reported recently[22], which exhibits straight large pore channels with circular openings of 18 rings along the c-axis interconnected
by a bidirectional system of 10 ring channels, yielding a structure with very large micropore volume (Fig 2, left) Although the synthesis conditions of ITQ-33 are easily accessible, they are not typical for silicogermanate zeolite analogs and were identified using high-throughput synthesis techniques ITQ-33 possesses the surface acidity capable of catalyzing with very high-activity, interesting catalytic reactions, such as alkylation of benzene with propylene to produce the industrially relevant cumene, while giving an extremely low yield of the undesirable n-propylbenzene
Fig 1 Representative SEM images of ethylene-containing hybrid mesoporous organosilica (Fig 3 from ref [13] ).
Trang 4(below 0.01% yield at 99% conversion) When working at very low
contact time (weight hourly space velocity or WHSV = 12 h1),
ITQ-33 decays much more slowly than the Beta zeolite
commer-cially used at present (Fig 2, right) Activity is maintained after at
least five reaction regeneration cycles (540 8C in air) ITQ-33 is very
active for alkylation and transalkylation of alkyl aromatics, as well
as for dealkylation of bulky alkylaromatics containing one or two
condensed aromatic rings
ITQ-33 is of interest for producing more diesel and less gasoline
while maintaining the propylene and butene yield, during catalytic
cracking of vacuum gasoil This issue is of importance, given that
diesel has a higher mileage efficiency than gasoline and given the
growing imperative to save fuel and restrict CO2emissions This
predicted catalytic behavior would be a consequence of the very
large pores that should increase the diesel yield, and the existence
of the 10-ring connecting pores that allow diffusion and cracking of
gasoline molecules, producing C3 and C4olefins ITQ-33 gives a
cracking conversion higher than Beta and close to that of a USY
(ultrastable Y) zeolite Furthermore, ITQ-33 produces more diesel,
less gasoline and propylene/propane, and isobutene/isobutane
ratios much higher than does USY, and very similar to results for
Beta zeolite Used together, ITQ-33 and ZSM-5 have an excellent
cooperative effect, giving a much higher diesel and propylene
yield, with lower gasoline yield, than does the combination of USY
and ZSM-5 zeolites used today If the stability and economics can
be further improved, ITQ-33 may serve as a suitable catalyst for
refining and petrochemical processes
Amorphous microporous molecular sieves with different pore
dimensions and topologies predefined by the size and shape of the
organic structure-directing agent were synthesized recently[23]
From the energetic diagram for the different steps occurring during
the synthesis of zeolites (Fig 3), it appears possible to synthesize in
an analogous way stable amorphous microporous molecular sieves
with pore dimensions predefined by the size and shape of the
organic structure-directing agent These synthesis steps are (a) an
induction period, (b) nucleation, in which viable nuclei are formed,
and (c) growth of the nuclei to form zeolite crystals Following the
reaction steps given in Fig 3, Corma and Dı´az-Caban˜ az [23]
synthesized and isolated, before nucleation occurred, stable
amorphous microporous materials These new microporous stable
materials can be considered as amorphous zeolite precursors (ZP)
containing zeolitic nuclei that are too small to show crystallinity by
different spectroscopic techniques These novel nanosized zeolitic
materials have several advantages over conventional zeolites: they
can always be obtained in high yields, with shorter synthesis times,
and in larger compositional ranges
Corma and Dı´az-Caban˜ az[23]also reported ZSM-12
(mono-dimensional 12-ring channel), NU-87 (bi(mono-dimensional 10-ring
pore), and ITQ-21 (three-dimensional 12-ring channels) zeolite
precursors that possessed micropore diameters and pore volumes
similar to corresponding well-crystallized zeolites These zeolite precursors correspond to partially arranged amorphous versions of the zeolite with already a large amount of Si–O–Si bonds formed, but with still an important fraction of internal defects, as was evidenced by 29Si MAS NMR 27Al MAS NMR showed that Al in calcined zeolite precursors is in tetrahedral coordination and gives raise to Brønsted acidity observed by pyridine-adsorption/deso-rption measurements These acid sites are active and selective for catalyzing alkylation and cracking reactions of hydrocarbons The catalytic results show that the microporous amorphous molecular sieve ZPITQ-21 gives the same catalytic activity as the correspond-ing large pore zeolite (12-R) ITQ-21 for crackcorrespond-ing 1,3-diisopropyl-benzene (DIPB), which can easily diffuse into 12-ring pore zeolites However, for cracking the bulkier 1,3,5-triisopropylbenzene (TIPB), ZPITQ-21 is more active than ITQ-21, showing the presence of slightly larger pores, probably due to incompletely formed cages and cavities in the former material The above observation is consistent with the distribution of the trimethylpentane isomers (TMP) obtained during alkylation of 2-butene with isobutane ZPITQ-21 is more selective towards the bulkier 2,2,4-TMP and 2,2,3-TMP (which have the highest octane numbers) than the corresponding ITQ-21 It has to be remarked that the molar ratio R (R = (2,2,4-TMP + 2,2,3-TMP)/(2,3,4-TMP + 2,3,3-TMP)) has been associated with shape selectivity effects in zeolites, being this ratio higher for zeolites with larger pores
3 Dehydrogenation and hydrogenation
There are a number of recent developments related to synthesis and application of novel metal nanoparticle and other
nanos-Fig 2 (Left) view of the ITQ-33 structure along [0 0 1], showing the 18-ring structure (oxygen atoms have been removed for clarity); (Right) percentage propylene conversion
as a function of time on-stream for ITQ-33 and Beta; ITQ-33 is represented by filled circles; Beta is represented by open circles T = 125 8C; P = 3.5 MPa; WHSV = 12 h 1 ; the benzene/propylene molar ratio is 3.5 (Figs 3 and 4, respectively, from ref [22] ).
Fig 3 Energetics of zeolite crystallization (Fig 1 from ref [23] ).
Trang 5tructured catalysts in two broad classes of heterogeneously
catalyzed organic reactions, dehydrogenation and hydrogenation
A wide spectrum of both dehydrogenation and hydrogenation
reactions is catalyzed by noble metal catalysts (e.g., palladium [Pd]
and platinum [Pt]) dispersed on metal oxide supports, whereas
dehydrogenation reactions, including oxidative dehydrogenation
reactions (ODH), are also catalyzed by supported transition metal
oxides, such as vanadium pentoxide (V2O5), molybdenum trioxide
(MoO3), and chromic acid (Cr2O3)
3.1 Dehydrogenation
Recently, McCrea and Somorjai[24]demonstrated that the rate
of cyclohexene hydrogenation and dehydrogenation is influenced
by the symmetry of the platinum single crystal faces They
concluded that the maximum turnover rate of hydrogenation
appeared at lower temperature than the dehydrogenation, and the
maximum hydrogenation rate was higher on Pt (1 1 1) while lower
on Pt (1 0 0) faces compared to the rate of dehydrogenation The
phenomenon was traced back to the difference of reaction
mechanisms on the two different surfaces and serves as an
excellent test reaction occurring with different rates over different
crystal faces
The same group reported for the first time the preparation of
catalysts with well-defined shaped metal (platinum, gold, silver,
etc.) nanoparticles in the pore system of ordered mesoporous
silicas In this method a colloid solution of metal nanoparticles
protected by organic molecules is used as template for the
synthesis of SBA-15 [25] and/or MCM-41 [26] mesoporous
silicates A novel type of catalysts composed of well-shaped
(cubic) metal particles and mesoporous matrix (pore size 7–8 nm)
were prepared and characterized by various physico-chemical
techniques The platinum nanoparticles have well-defined shapes,
such as cubic, tetrahedral, cubo-octahedral, etc The cubic particles
have (1 0 0) faces, the tetrahedral particles display (1 1 1) faces,
and the cubo-octahedral particles have both type of faces In the
most favored case, the platinum nanoparticles might be embedded
in the pores of mesoporous silica structures, such as MCM-41 and
SBA-15 The catalysts prepared in this way may provide a rather
direct link to test insights gained in surface science (UHV) studies
done on single crystals
Such nanostructured catalytic materials confined to cavities
and pores of regular nanoscale dimensions are the subject of
continuing interest because they have unique size-dependent
catalytic properties, which are significantly different than those of
the corresponding bulk catalysts [27] It was recently
demon-strated that Pt nanoparticle catalysts for methylcycloxane (MCH)
dehydrogenation may be prepared by a ‘‘one-pot’’ encapsulation
into the mesopore channels of ordered mesoporous SBA-15 silica
[28] The fabrication of metallic Pt nanoparticles with controllable
size and shape has become an important topic in nanotechnology
owing to their unique catalytic performance The use of ordered
mesoporous materials as hosts to limit the growth of
nanos-tructured materials in their pores is a highly promising approach to
stabilize metal nanoparticles against undesirable aggregation and
sintering Preparation of Pt nanoparticles or nanowires in porous
materials typically involves at least two or three steps to restrict Pt
nanoparticles in the mesoporous matrix Chen et al.[28]developed
a novel one-pot approach to directly introduce Pt nanoparticles
into the mesochannels of SBA-15, which required no preexisting
mesoporous host and Pt nanoparticles, and no extra reduction
process of the platinum precursor (Fig 4)
This co-assembly method is based on the I+MS+ scheme for
the synthesis of mesoporous host [29] The positively charged
surfactants such as protonated block copolymers [S+] and cationic
inorganic oxide precursors [I+] are assembled together through the
mediator [M] 3-Mercaptopropyltrimethoxysilane [MPTMS] with thiol groups are added to modify the cationic precursors [I+] in order to confine the Pt nanoparticles inside the mesochannels because platinum ions are easy to combined with the thiol groups through strong chemical bonds The mediator in this case could be the anionic platinum complex and chloride ions.Fig 5shows the total synthesis route to obtain monodisperse Pt nanoparticles in SBA-15 The direct co-assembly of MPTMS and tetraethylortho-silicate (TEOS) is preferred over the commonly used post synthesis grafting between surface silanols and functional silylation agents, which provides for a more homogeneous distribution of organic ligands in the framework After adsorption of platinum ions onto the thiol groups, platinum sulfide analogues can be decomposed into metallic platinum and sulfur oxides when heating in air providing a homogeneous distribution of metallic Pt nanoparticles
in the mesochannels of SBA-15 after calcination at 550 8C to remove the template (Fig 5)
Fig 4 Preparation of highly dispersed Pt nanoparticles in the SBA-15 mesochannels (Scheme 1 from ref [28] ).
Fig 5 TEM images and particle size distribution of Pt–SBA-15 (Fig 5 from ref [28] ).
Trang 6These nanocomposites exhibited higher catalytic activity and
stability than conventional Pt/SiO2 catalysts in the
methylcyclo-hexane dehydrogenation (Fig 6) All samples showed nearly 100%
selectivity to toluene over the entire experimental run The initial
MCH conversion was ca 65.0% for all catalysts, which then
decreased with time on-stream Pt confined in SBA-15 displayed
higher dehydrogenation conversion and stability than that
supported on SiO2 under the same reaction conditions The
smaller particle size and more homogeneous dispersion of Pt in
SBA-15 may result in its higher catalytic stability in MCH
dehydrogenation as the confinement of ordered mesochannels
restricts further growth of Pt nanoparticles during the
dehydro-genation reaction
Recent investigations of Fe substitution into the inorganic walls
of SBA-15 have revealed that Fe location had a strong influence on
the selectivity to dehydrogenation and dehydration of ethanol
[30–32] At low Fe loading, Fe was present as isolated species in the amorphous SBA-15 silica and formed aggregated clusters of iron oxide at high Fe loading The isolated Fe species possessed Brønsted acidity that resulted in selective formation of ethylene, whereas the Fe clusters were efficient in the formation of ethylene and acetaldehyde
Besides dehydrogenation catalysis, SBA-15 has been explored further as a catalytic support to prepare a novel interfacial hybrid epoxidation catalyst by a new immobilization method for homogeneous catalysts [33] This approach involved coating SBA-15 support with an organic polymer film containing active sites The titanium silsesquioxane (TiPOSS) complex, which contains a single-site titanium active center, was immobilized successfully by in situ copolymerization on a mesoporous SBA-15-supported polystyrene polymer The resulting hybrid materials exhibit attractive textural properties, such as highly ordered mesostructure, large specific surface area (>380 m2g1) and pore volume (0.46 cm3g1), and high activity in the epoxidation of alkenes In the epoxidation of cyclooctene with tert-Bu hydrogen peroxide (TBHP), the hybrid catalysts have rate constants comparable to that of their homogeneous counterparts and can
be recycled at least seven times These immobilized catalysts can also catalyze the epoxidation of cyclooctene with aqueous H2O2as the oxidant In two-phase reaction media, the catalysts show much higher activity than their homogeneous counterparts due to the hydrophobic environment around the active centers They behave
as interfacial catalysts due to their multifunctionality, i.e., the hydrophobicity of polystyrene and the polyhedral oligomeric silsesquioxanes (POSS), and the hydrophilicity of the silica and the mesoporous structure The simultaneous immobilization of homogeneous catalysts on two conventional supports (inorganic solid and organic polymer) has been shown to provide novel heterogeneous catalytic ensembles with attractive textural prop-erties, tunable surface propprop-erties, and optimized environments around the active sites
Several novel synthesis approaches to surface immobilization
of homogeneous catalysts in porous or high-surface-area supports
Fig 6 Methylcyclohexane conversion by Pt-SBA-15 and conventional Pt-SiO 2
catalysts at atmospheric pressure, 300 8C and WHSV of 27.1 as a function of time
on-stream (Fig 6 from ref [28] ).
Trang 7(Fig 7) have been reported by Tada and Iwasawa[34] Interfacial
chemical bonding of Pd monomers with Pd–P (P: P(O–iPr)3,
PMe2Ph, and dppf) and Pd–N (N: tmeda, methylpiperidine, and
cyclohexylamine) to SiO2, Al2O3, and TiO2surfaces was employed
to obtain a series of supported Pd complex catalysts for the
hydroamination of 3-amino-propanol vinyl ether The order of the
hydroamination activities observed for these catalysts correlated
with the pKavalues of the SiO2, Al2O3, and TiO2surfaces The most
ionic bond, Pd–OSi, was favorable for the hydroamination of
alkenes, while the Pd–OAl bond with relatively more covalent
character did not efficiently promote the reaction These results
show that the chemical bonding with surfaces has a major impact
on catalytic activity, and the nature of the support is an important
parameter that can be explored to produce new catalytic behavior
The chiral self-dimerization of supported vanadium complexes
on a SiO2 surface, which is a novel phenomenon for metal
complexes on oxide surfaces, has been reported recently Two
V-monomer complexes with Schiff-base ligands spontaneously
dimerized via a selective reaction with a surface Si–OH group,
and the formed V dimer had a unique chiral conformation, which is
highly enantioselective for the asymmetric oxidative coupling of
2-naphthol with 96% conversion, 100% selectivity to 1,10-binaphthol
(BINOL), and 90 ee% This surface complex is the first
hetero-geneous catalyst for the asymmetric coupling reaction, whereas
the V monomer is inactive for the oxidative coupling Another
technique reported by the Iwasawa group, surface
functionaliza-tion of the SiO2 support with achiral
3-methacryloxypropyl-trimethoxysilane, remarkably amplified the enantioselective
catalysis of SiO2-supported Cu–BOX complexes for asymmetric
Diels–Alder reaction BOX (bis(oxazoline)) is one of the practical
ligands for asymmetric catalysis Enantioselectivity for Diels–Alder
reaction can be significantly regulated by surface functionalization
with achiral silane-coupling reagents on SiO2-supported Cu–BOX
complexes
Molecular imprinting methods have recently showed
signifi-cant promise in creating template-shaped cavities with memory of
template molecules that are reminiscent of artificial enzymes
possessing recognition ability for particular substrate molecules
[35] Acid-base catalysts and metal complexes synthesized by
molecular imprinting techniques provide promising molecular
recognition catalysis with 100% selectivity for a variety of catalytic
reactions where natural enzymes cannot be employed[34]
Molecular imprinting typically consists of several steps: (1)
attachment of a metal complex on robust supports, (2)
surround-ing of the metal complex by a polymer matrix, and (3) production
of a shape selective cavity on the metal site in the matrix Most of
the imprinted metal-complex catalysts have been prepared by
imprinting in bulk polymers, which discourages the access of
reactant molecules to the active sites in the bulk Moreover,
polymers tend to be unstable in organic solvents or under
demanding catalytic conditions, such as in the presence of
oxidants, at high temperatures, etc To overcome these limitations,
the Iwasawa group designed molecular-imprinted catalysts for
oxide-supported metal complexes to produce shape-selective
reaction sites by using a ligand on a metal center as a template
A ligand of a supported metal complex not only influences its
catalytic activity but also provides an unsaturated, reactive metal
site with a ligand-shaped space after removal of a ligand
The Iwasawa group chose a ligand of the attached metal
complex with a similar shape to a reaction intermediate
(half-hydrogenated alkyl) of alkene hydrogenation as a template This
strategy to design active and selective catalysts was based on the
following five factors for regulation: (1) conformation of ligands
coordinated to a rhodium (Rh) atom, (2) orientation of vacant site
on Rh, (3) cavity with complementary molecular shape for the
reaction space produced after template removal, (4) architecture of
the cavity wall, and (5) micropore in inorganic polymer-matrix overlayers stabilizing the active species at the surface A P(OCH3)3
ligand was used as a template with a similar shape to one of the half-hydrogenated species of 3-ethyl-2-pentene, which can produce the template (reaction intermediate)-shaped cavity after extraction of the ligand
The Iwasawa group succeeded in preparing Rh monomer and a pair of Rh monomers by using appropriate precursors on the surfaces On both surfaces, Si(OCH3)4was deposited by chemical vapor deposition and converted into SiO2-matrix overlayers surrounding the attached Rh complexes via a hydrolysis– polymerization step Finally, the template ligand, P(OCH3)3, was extracted from the attached Rh complex in the SiO2-matrix overlayers yielding the molecularly imprinted Rh-monomer and Rh-dimer catalysts The homogeneous complexes, Rh2Cl2(CO)4and RhCl(P(OCH3)3)3, and the supported species, Rh2Cl2(CO)4/SiO2, showed no activity for alkene hydrogenation at 348 K On the other hand, the molecular-imprinted catalysts exhibited significant catalytic hydrogenation activities under similar reaction condi-tions For example, hydrogenation of 2-pentene on the molecularly imprinted Rh-dimer catalyst was promoted 51 times as compared
to that on the supported catalyst The metal–metal bonding and coordinative unsaturation of the Rh dimer are key factors for the remarkable activity of the imprinted Rh-dimer catalyst The selectivity for the alkene hydrogenation on the molecularly imprinted catalysts depended on the alkene size and shape which should come into the reaction site in a template cavity in addition
to the electronic and geometric effects of the ligands
The location of the Rh center for alkenes coordination, the conformation of the remaining P(OCH3)3ligand, the orientation of the template vacant site on Rh, the template-shaped cavity, the architecture of the cavity wall, and the micropore surrounding the
Rh dimer in the SiO2-matrix overlayers provided active imprinted catalysts for the size- and shape-selective alkene hydrogenation Therefore, the arrangement of active sites on surfaces by chiral self-dimerization, surface functionalization with achiral reagents, and molecular imprinting provide new powerful approaches for the design of selective catalyst surfaces in three dimensions beyond conventional homogeneous and heterogeneous catalyst systems
3.2 Hydrogenation
The use of well-defined metal nanoparticles (1–10 nm) for catalytic processes is a rapidly growing area[36–39] Similarly to molecular complexes, metal nanoparticles have been proved to be efficient and selective catalysts for hydrogenation of olefins or C–C couplings, but also for reactions that are not catalyzed or are poorly catalyzed by molecular species, such as hydrogenation of arenes However, despite impressive progress in asymmetric catalysis, few colloidal systems have been found to display an interesting activity
in this field Those systems that show promise include Pt(Pd)/ cinchonidine for the hydrogenation of ethyl pyruvate and Pd-catalyzed kinetic resolution of racemic substrates in allylic alkylation
Metal nanoparticle catalysts can be obtained by a variety of methods according to the organic or aqueous nature of the media and the stabilizers used[39] Poly(vinyl)pyrrolidone, PVP, has been the homopolymer most used as stabilizing agent for metallic nanoparticles Others like cellulose and polysaccharide, polyviny-lalcohol, polystyrene, polyacid or poly(vinyl)formamide deriva-tives, and copolymers and various dendrimers have been also applied for similar purposes Side-chain functionalized polymers for their applications as stabilizers of metallic nanoparticles are particularly attractive, because the functional groups can interact with the metallic surface[40] Favier and colleagues functionalized
Trang 8poly(methyl vinyl ether-co-maleic anhydride) and employed it to
stabilize 3–20 nm sized Pd, Pt and Rh nanoparticles, which were
investigated in catalytic hydrogenation of ethyl pyruvate (Rh
nanoparticles) and C–C coupling using phenylboronic and
2-methylnaphtyl-1-yl boronic acids (Pd nanoparticles) The Rh
nanoparticle catalysts employed in the hydrogenation reaction
were significantly more active than the conventional
heteroge-neous Rh catalyst and resistant to agglomeration and sintering
However, organic polymers are not very stable at high
temperatures, and the catalytic reactions in solution may be
accompanied by some swelling, which can cause significant
mass-transfer resistances Therefore, the use of inorganic supports or
porous hosts appears to be more suitable Paˆrvulescu et al.[41]
reported SiO2embedded Pd, gold (Au), and highly alloyed Pd–Au
colloids prepared by sol–gel embedding of presynthesized colloids
and their performances in the hydrogenation of several substrates:
cinnamaldehyde, 3-hexyn-1-ol, and styrene These data showed
evidence that alloying Pd with Au in bimetallic colloids leads to
enhanced activity and most importantly to improved selectivity
Moreover, the combination of the two metals resulted in catalysts
that were very stable against poisoning, as was observed for the
styrene hydrogenation in the presence of thiophene
An interesting example of metal nanoparticle systems prepared
by chemical vapor deposition of volatile metal organic precursors
into the large pores of metal organic frameworks (MOF) has been
reported by Hermes et al [42] They have synthesized several
metal@MOF systems in which Cu and Pd nanoparticles were grown
inside the pores of MOF-5 framework and were found to be active
catalysts for methanol production from syngas (Cu@MOF-5) and
cyclooctene hydrogenation (Pd@MOF-5), whereas in the case of
the Au@MOF-5 system, Au atoms migrated from the pore cavities
of MOF-5 to the external surface and aggregated into large 20 nm
Au particles, which were inactive in CO oxidation
Wilson et al [43] investigated a particle size effect for
hydrogenation over unsupported Pd nanoparticle catalysts in a
size range (1.3–1.9 nm) that has not been widely studied and
demonstrated that the rate of hydrogenation of allyl alcohol is a
function of the diameter of the Pd nanoparticles Furthermore, kinetic data indicated that this effect is probably electronic in nature for particles having diameters <1.5 nm, but for larger particles it depends primarily on their geometric properties They employed dendrimer-encapsulated nanoparticles (DENs) prepared using dendrimer templates that exert a high degree of control over the size, composition, and structure of catalytically active nanoparticles in the <3 nm size range Specifically, sixth-genera-tion, hydroxyl terminated polyamidoamine dendrimers (G6-OH) were used to synthesize Pd DENs containing an average of 55, 100,
147, 200, or 250 Pd atoms (G6-OH[Pdn], where n is the average number of atoms per particle)
Alkene hydrogenation occurs via the Horiuchi–Polanyi mechanism, which involves dissociative adsorption of H2 onto the catalyst surface, followed by stepwise hydrogenation of the
C C double bond[44].Fig 8shows that only the total number of face atoms increases with particle size, while the numbers of surface and defect atoms both decrease Differences in reaction rates as a function of catalyst size arise from either electronic or geometric effects For example, as the size of a nanoparticle decreases, its electronic properties change from those of a metal to
an insulator and then to something akin to those of a molecule, which modulate the catalytic properties of nanoparticles Geo-metric effects are most evident when a reaction requires a specific type of surface atom, because the ratio of defect (vertex and edge)
to face atoms changes dramatically as a function of size for <5 nm diameter particles While geometric effects have been observed for homogeneous colloidal Pd catalysts for both the Heck and Suzuki coupling reactions, literature reports pertaining to size effects in hydrogenation reactions focus almost exclusively on supported (heterogeneous) Pd catalysts[43]
There is a major conceptual difficulty in that, in the size range where the electronic structure of metal particles is changing, there are also major changes in the geometric arrangement of atoms on the surface [45–47] Therefore, it is impossible to assign the catalytic effects of particles of size less than 5 nm unambiguously
to either a geometric or an electronic effect However, if the size
Fig 8 (a) Plot of the rate of hydrogen consumption as a function of particle diameter (b) Plot of the numbers of surface, defect, and face atoms for each particle size The data
Trang 9effect persists for metal particles larger than 5 nm, it is more likely
due to a requirement of a specific active site, since the changes in
electronic structure become insignificant for such larger particles
In addition to catalyst size, the preparation and structure of
nanoparticles are also important factors that must be taken into
account when comparing catalytic activity This is because
preparation methods, stabilizing ligands, and polydispersity can
lead to activity changes that may mask true particle size effects For
example, Pd nanoparticles having similar diameters, but stabilized
by either poly(vinylpyrrolidone) or 1,10-phenanthroline, exhibit
very different catalytic activities for the hydrogenation of
1,3-cyclooctadiene [48] The hydrogenation reaction is sensitive to
both the electronic and geometric properties of the catalytic Pd
nanoparticles, both of which change quickly in the 1.3–1.9-nm
diameter size range The hydrogenation kinetics of allyl alcohol
[42]are dominated by electronic effects for the smallest particles
(<1.5 nm diameter) and by geometric effects for larger particles
(1.5–1.9 nm diameter) Results of the type described here were
enabled by the high degree of monodispersity resulting from the
dendrimer templating approach to nanoparticle synthesis
The synthesis of particles with narrow size distribution and
homogeneous physicochemical properties in order to establish
reactivity–morphology relationships is a highly desirable but
challenging effort Controlling the shape at the nanoscale
translates into ability to control the relative exposure of different
crystal facets and the number of atoms on corners and edges and,
hence, ability to tune the activity and selectivity of a catalytic
system The application of shape-controlled synthesis of metallic
particles in catalysis is a relatively new direction However, the
number of studies using such catalytic systems have been rapidly
growing in recent years Fukuoka et al [49] reported a direct
comparison between catalytic properties of Pt nanospheres and
nanowires synthesized inside a mesoporous template In the
hydrogenolysis of butane, Pt nanowires exhibited a 36 times
higher turnover frequency than spherical particles Moreover, Pt
nanowires were found to produce ethane by secondary–secondary
carbon bond cleavage, whereas spherical Pt particles were able
only to cleave terminal C–C bonds and then to produce methane
and propane This change of catalytic properties was ascribed to
the preferential exposure of (1 1 0) planes and/or to more
electron-deficient sites Park et al.[50]reported tetrahedral Rh
nanopar-ticles preferentially exposing the catalytically active (1 1 1) planes
that were prepared by thermal decomposition of Rh complexes in
oleylamine and supported on charcoal These supported
(4.9 0.4) nm Rh particles showed a 5.8- and 109-fold increase in
activity for the hydrogenation of anthracene compared to spherical
Rh nanoparticles and commercial Rh/C catalyst, respectively
Mertens et al [51] recently reported Au0 nano-colloids stabilized by polyvinylpyrrolidone (PVP), which displayed high chemoselectivity in the hydrogenation ofa,b-unsaturated alde-hydes and ketones to allylic alcohols Analogous Pt0and Ru0 nano-colloids were more active than Au0, but substantially less chemoselective for allylic alcohols Experimental control over the Au0cluster generation provided the opportunity to investigate the size-dependent catalytic behavior of nano-Au0and determine the optimum gold cluster size leading to the highest allylic alcohol yields The optimum cluster size of the Au0colloids obtained using HAuCl43H2O as the metal precursor and various PVP/Au ratios were determined for the highest crotyl alcohol yield At the optimum, the PVP/Au ratio was 6, and based on TEM, nearly spherical Au0colloids have a mean diameter of 7 nm
Telkar et al.[52]compared the catalytic properties of spherical and cubic Pd particles for the hydrogenation of butyne-1,4-diol and
of styrene oxide Very high turnover frequencies were found in both cases using cubic nanoparticles Moreover, cubic particles were found to be more selective than spherical particles for the hydrogenation of butyne-1,4-diol into but-2-ene-1,4-diol Simi-larly, the selectivity in 2-phenyl ethanol for the hydrogenation of styrene oxide was nearly 100% on cubic particles versus less than 50% for conventional catalytic systems The most detailed study was undertaken by Narayanan and El-Sayed[53,54] They used a model reaction, the electron transfer reaction between hexacya-noferrate(III) and thiosulfate ions to study the catalytic properties
of cubic, tetrahedral and spherical Pt particles The particles’ reactivity follows the increasing order: cubes < spher-spheres < tetrahedra The higher reactivity of tetrahedral nano-particles was related to a higher proportion of corner and edge sites
on these particles exposing only {1 1 1} faces Therefore, the morphological control during the synthesis of nanoparticles seems
to lead to highly selective catalysts, due to preferential exposure of crystallographic planes Most of these studies were performed using polymers (polyvinylpyrrolidone, N-isopropylacrylamide, polyacrylate) These organic ligands can modify catalytic proper-ties by affecting the electronic structure of Pt nanoparticles as well
as by limiting the accessibility of active sites to reactants and intermediates[52,55]
Berhault et al successfully employed structure-directing agents and a so-called ‘‘seeding-mediated’’ approach to prepare stable and surfactant-free anisotropic Pd nanoparticles (Fig 9) These were investigated in selective hydrogenation of buta-1,3-diene[56] In this method, anisotropic growth of nanoparticles is controlled by the selective adsorption of structure-directing agents, e.g., ions, surfactants, or polymers The selective adsorption of a structure-directing agent inhibits the growth of particles along a given
Fig 9 TEM images of anisotropic Pd nanoparticles: (A) nanorods and polyhedra; (B) a typical nanorod with a five-fold symmetry; (C) a polyhedron formed from six
Trang 10crystallographic direction through a preferential stabilization of
specific faces The polyol process was extensively used to
synthesize Group VIIIb noble metal-based nanostructured
cata-lysts The polyol process uses a polymer as structure-directing
agent (generally, PVP), and reduction is performed by the alcoholic
solvent In aqueous media, surfactants (mainly
cetyltrimethylam-monium bromide, CTAB) play the role of structure-directing agent
Synthesis in aqueous solutions is particularly attractive in the way
that (1) surfactant can be eliminated easily from the surface of the
nanoparticles and (2) using a ‘‘seeding-mediated’’ approach, the
respective rates of nucleation and growth could be better
controlled through the injection of seeds serving as unique centers
of nucleation-growth at the beginning of the growth process
Berhault and coworkers applied this methodology to the
synthesis of anisotropic Pd nanoparticles that they employed in
the selective hydrogenation of buta-1,3-diene Selective
hydro-genation of unsaturated compounds is a technologically important
process in heterogeneous catalysis Dienes are unwanted
by-products of thermal cracking of petroleum cuts Depending on the
target objective, buta-1,3-diene should be converted selectively
into but-1-ene in the course of the production of polymers, or into
but-2-ene for producing petrochemicals (higher olefins, alcohol),
or into gasoline of high octane number after alkylation
Palladium-based catalysts are frequently used for the selective hydrogenation
of buta-1,3-diene This reaction is structure-sensitive, and studies
on model catalysts have shown that Pd (1 1 0) exhibits a higher
selectivity for the formation of butenes than Pd (1 1 1)[57,58]
Similarly, Guo and Madix[59]have also observed that Pd (1 0 0)
surfaces are particularly selective for the hydrogenation of dienes
into alkenes without further hydrogenation into alkanes
Berhault et al [56] successfully applied a seeding-mediated
approach to synthesize well-defined morphological Pd
nanopar-ticles (nanocubes, nanotetrahedra, nanopolyhedra, and nanorods)
using cetyltrimethylammonium bromide (CTAB) as both capping
and structure-directing agent (Fig 10) Pd nanorods were found to
expose preferentially (1 0 0) lateral planes due to the selective
inhibition by CTAB of the crystalline growth of {1 0 0} facets After
deposition ontoa-Al2O3, these nanostructured Pd particles were
tested in the selective hydrogenation of buta-1,3-diene at 298 K
under 20 bars of H2 These anisotropic catalysts are highly selective
catalysts for the hydrogenation of buta-1,3-diene into butenes
without further hydrogenation to butane As compared to an isotropic Pd catalyst, anisotropic Pd catalysts were found to be highly selective for the hydrogenation of buta-1,3-diene into butenes without further hydrogenation into butane Moreover, further hydrogenation of but-1-ene into butane over these nanostructured catalysts was reduced, whereas but-2-enes were hardly converted This would be related to the higher exposition of {1 0 0} facets on the Pd nanorod catalyst in the absence of any selective poisoning effect due to CTAB Finally, an unusually high level of cis but-2-ene isomer was observed, suggesting a modification of the mechanism usually accepted for Pd catalysts when a well-defined nanomorphology is achieved The mechanism for the hydrogenation of buta-1,3-diene on the nanostructured catalysts differs from the one generally assumed on ‘‘more classical’’ isotropic catalysts with a higher cis/trans but-2-ene ratio, probably resulting from an interconversion between syn and anti di-adsorbed butadiene species (Fig 10) Therefore, nanos-tructured Pd catalysts present interesting and new catalytic properties differing considerably from the conventional isotropic catalysts They are also useful model catalysts in order to bring new mechanistic information for selective hydrogenation reactions under relevant experimental conditions
4 Selective oxidation
Selective oxidation processes represent a large class of organic reactions where the development of clean and efficient ‘‘green chemistry’’ processes can have a significant positive economic and environmental impact to mitigate the accumulation of greenhouse gases in the atmosphere and other environmental concerns resulting from the world’s growing consumption of fossil fuels and current fuel processing techniques Selective oxidation processes leading to chemical intermediates and fine chemicals have up to now largely relied on stoichiometric reactions employing chromate, permanganate, and renate species Simple catalytic reactions making use of molecular oxygen or hydrogen peroxide that require minimal energy and produce minimal by-product waste are highly desirable in order to replace stoichio-metric reactions that are expensive and environmentally unfriendly
4.1 Selective oxidation catalysis by nanosized gold and other noble metals
A key emerging technology that shows great promise in selective oxidation is catalysis by gold and gold-containing nanoparticles and nanoclusters [60,61] Gold was earlier con-sidered to be chemically inert and regarded as a poor catalyst However, when gold is highly dispersed on metal oxides as small nanoparticles with diameters of less than 10 nm (and preferably in the 1.5–3 nm range), it becomes a highly active oxidation catalyst for many reactions, such as CO oxidation and propylene epoxida-tion in the gas phase[62] Most recently, polymer-supported gold has also shown great promise as a catalyst for green liquid-phase selective oxidation processes[63] Higher catalytic reactivity was achieved for the polymer-supported catalysts than for gold supported on metal oxides via simultaneous optimization of the support, solvents, and size of gold particles
Over the last decade, following the pioneering work by Prati and Rossi[64], much attention also has been paid to the development
of liquid-phase oxidation processes employing gold nanoparticle catalysts The ultimate objective is to carry out green liquid-phase oxidations at atmospheric pressure and room temperature, either
in aqueous media or under solvent-free conditions, and using air as oxidant Gold nanoparticles supported on activated carbon or metal oxides are active for liquid-phase selective oxidation of
Fig 10 Proposed reaction network for buta-1,3-diene hydrogenation on the
nanostructured Pd catalysts The dotted crosses indicate unfavorable routes for the
nanostructured Pd catalysts as compared to the isotropic catalyst, whereas solid
crosses indicate prohibited routes (Scheme 1 from ref [56] ).